Torrefaction (i.e., pyrolysis) is the thermochemical treatment of biomass that is typically carried out under atmospheric pressure in an anaerobic environment. During torrefaction, water and volatiles that are contained in the biomass are removed. Also during this process, biopolymers including cellulose, hemicellulose, and lignin partly decompose. The resulting product from torrefaction is a solid, dry, blackened material that is often referred to as “torrefied biomass,” “torrefied biofuel,” or “bio-coal”. Torrefied products such as torrefied biomass have numerous industrial uses but are typically used as an alternative to fossil fuels because of their chemical properties (e.g., combustibility), low SO2 emissions, and renewability. Also, legislative advantages exist for burning renewable fuels such as torrefied biofuels because these fuels count as zero when calculating CO2 emissions. Thus, burning these renewable fuels are advantageous for at least these reasons.
Generally, biofuels can be divided into two classes, and these classes include first generation and second generation biofuels. First generation biofuels are derived from materials that can be used as food, and examples of first generation biofuels can include, but are not limited to, methanol produced from corn, diesel fuels produced from palm oil, and ethanol produced from either corn or sugar cane. However, unlike first generation biofuels, second generation biofuels are derived from materials that cannot be used as food by humans and/or livestock. Examples of second generation biofuels include, but are not limited to, cellulosic materials such as wood and grasses. In certain aspects, the biomass materials (e.g., woods and/or grasses) can first be first torrefied, and if desired, the biomass materials can be further distilled until little remains of the original material except for ash.
During the torrefaction process, biomass typically loses approximately 20% to 30% of its drymass, while only 10% of the energy content in the biomass is lost. This energy (e.g., in the form of volatiles) can be used as a heating fuel during the torrefaction process or it can be processed to recover the organic compounds. After the biomass is torrefied, it can be compacted and densified, usually into briquettes or pellets using conventional densification equipment. These processes further increase the density of the torrefied material and improve the torrefied material's hydrophobic properties. When densified, the final torrefied product repels water and thus can be stored in most places and conditions including moist air and rain.
To carry out torrefaction, numerous dryers have been designed, and examples of such dryers include, but are not limited to, Wyssmont's multi-hearth Turbo Dryer® and the Agri-Tech dryer. FIGS. 1, 2, and 3 show various perspectives of an exemplary, vertically arranged multi-hearth torrefaction dryer (10) and the components that it is comprised of. This device has a biomass inlet (14) and a biomass outlet (16) and a series of vertically arranged rotatable trays (12) with fixed sweepers (36) and height controllers (34) and a central air distribution fan (25). As the trays rotate the material (e.g., biomass), the material being dried or torrefied falls vertically through large gaps (e.g., approximately 4″ to 6″) or slots (38) in the trays onto another tray below. In addition, as shown in FIGS. 1 and 2, externally heated vapors enter the dryer through vapor inlets (24). These externally heated vapors aid in drying the solid materials being processed, and in specific instances, if high vapor temperatures are used, the solids can be torrefied. Furthermore, the device of FIGS. 1 and 2 have multiple, non-separated treatment zones created by the segmented nature of the center fans (25) connected to the center shaft (32). These center fans help create horizontal flow patterns. However, because the vapors and center fans create great turbulence inside the dryer, a large annular space as shown in FIG. 2 is required between the leading edge of the trays and the inner wall of the dryer. As indicated above, the material is transferred from an upper tray to a lower tray and is also dried and/or processed by the externally heated vapors. Eventually the final product is discharged through a biomass outlet (16), and the vapors can be discharged through the vapor outlet (26).